While many Americans sweat income tax time, for billionaires, it's barely on the calendar. That's because for the very richest, income plays a minor role in wealth. To lower tax liabilities, billionaires reduce their actual income to a trickle, while living off the fortune they’ve amassed in appreciating assets: stocks, real estate, and the often rapidly growing companies they own. A study released last summer by professors at the University of California, Berkeley, found that the all-in tax rate paid b ...